This invention relates to multilayer interference filters formed by vapor deposition onto a moving substrate, and more particularly relates to a method and apparatus for controlling the thickness distribution of the filter layers along the direction of movement.
U.S. Pat. No. 4,683,398 describes a cathode ray tube for projection color television having a multilayer interference filter between the glass face panel and the luminescent screen. Such a filter results, among other things, in significantly greater brightness of the luminescent output of the tube.
Such interference filters are typically composed of alternating layers of materials having a high and a low index of refraction, respectively. The layers are preferably formed by vapor deposition onto the inner surface of the glass face plate.
Mass production of such filters is carried out in a vacuum chamber containing the source materials to be evaporated, means for heating the source materials, and a dome-shaped fixture adapted for holding a multiplicity of glass face plates. The plates are arranged in rows, each row forming a circle around the dome, so that each plate is approximately equidistant with the other plates from the source materials.
The dome is rotated during deposition, not only to promote uniform distribution of the deposited material on the plates, but also to pass the plates behind one or more stationary dodgers located between the source materials and the plates. These dodgers are designed to result in increasing thickness of the layers toward the edges of the plates, which has been shown to result in even greater increases in brightness than can be obtained for a uniform thickness distribution.
Ideally, the thickness of the layers should increase in all directions from the center of the plate. Unfortunately, however, such an arrangement as described above can only be used to control thickness distribution in a direction normal to the direction of movement of the plates.
A method is known by which a radial thickness distribution can be achieved. This so called "planetary" method involves rotating each plate about an axis normal to the surface of the plate, while the plate passes behind the dodger as described above. However, in addition to adding to the complexity of the apparatus, the rotating plates require significantly more space in the dome than do stationary plates. Thus, the throughput of the apparatus is significantly reduced. Also, in the case of skirted panels, the planetary drive is adequate only for circularly symmetric plates or "discs", and not for rectangular plates.